For the public record: Nope. Turns out @Monal doesn't do OMEMO:2 at all, and I'm the one doing it wrong. The key I thought belonged to Monal actually belongs to me, created in an earlier round of testing that I forgot about, added to the fact that comparing key fingerprints between multiple devices with a screen reader is really, really annoying. God I can't wait until I'm done thinking about encryption for ever and ever.